1. Field of the Invention
The present invention relates to a flame-retardant copolyester which is useful as a general industrial material necessitating flame retardance, particularly as a coating material for an electric wire, and a composition containing the same. Particularly, it relates to a flame-retardant halogenated copolyester which contains a branched or crosslinked structure and which does not lose its softness by thermal history and is excellent in flame retardance, a composition containing the same and an electric wire coated with the same.
2. Description of the Prior Art
Up to this time, rubber, polyvinyl chloride, polyethylene, polypropylene, nylon and the like have been used as a coating material for an electric wire. Particularly, polyvinyl chloride has been often used from the viewpoint of flame retardance and mechanical strengths. As the service environment of such a coating material has become severe, the level of the characteristics required with respect to the material has also enhanced. Namely, it has been required that the material be excellent in heat resistance, electrical characteristics and flame retardance and can exhibit excellent capability for forming a coating thin enough to attain space saving.
Although both a fluororesin and a or crosslinked polyethylene satisfy these requirements, both and unsatisfactory in their individual capability for forming a thin coating, and particularly, a fluororesin is further disadvantageously expensive.
Polyethylene terephthalate and polybutylene terephthalate are noticed by virtue of their high capability for forming a thin coating and their having excellent mechanical strengths (softness and wear resistance), heat resistance and electrical characteristics. However, they are insufficient in flame retardance, so that various attempts have been made in order to improve their flame retardance. Among the attempts made, is by introducing a halogen compound as a comonomer into a polyester which is free from the bleeding of a flame retardant. This halogenated copolyester is promising as a coating material for an electric wire.
However, such a polyalkylene terephthalate copolyester is a crystalline resin, so that it exhibits significant lowering in its softness by thermal history such as thermal treatment or service in an heated state to bring about a lowering in the mechanical strengths such as impact resistance. Therefore, the use of the copolyester in the vicinity of a heat sourse or in an environment in danger of heat buildup must be avoided, thus restricting the application thereof considerably.
An attempt of adding an elastomer to such a copolyester to thereby reduce the crystallinity as much as possible has been made for the purpose of overcoming the above disadvantage.
Although the composition according to this attempt is slightly improved in its qualities, the composition still contains crystalline resin matrix, so that it has disadvantages in that it is intolerable to long-term thermal history and that the lowering in the crystalline resin content of the composition brings about a lowering in the mechanical characteristics such as wear resistance.